The use of ultra high purity of chemicals, in particular of chemical gases, used in the processing of electronic materials (known as electronic specialty gases or ESG) is known to be a key factor in achieving acceptable production yield, production reproducibility and high quality in the manufacturing of electronic devices. For a recent review, see e.g., M. Liehr and G. W. Rubloff, J. Vac, Sci, Technol. B 12, 2727 (1994) which is hereby incorporated by reference.
Even if high purity is achieved at the manufacturing and purification step of such chemical gases, it is known that contamination occurs easily during their flowing to the point of use, through an equipment network (also referred to as a gas distribution network) which can include large lengths of tubing and many components for the control of pressure and flow rate (e.g. pressure reducers, valves, mass-flow controllers, filters, purifiers, etc.) which have to be interposed between the source (e.g. a gas cylinder in a gas cabinet) and the point of use (e.g. the process reactor) as well as other components well known to those of ordinary skill in the art.
Therefore, elaborate development efforts have been devoted to the optimization of the materials, surfaces, assembling techniques and design of line construction in order to achieve a minimal developed area to which the gases will be exposed. Such developments include electropolished metal surfaces, the absence of deadspaces in the distribution line design. Other developments have been directed to minimizing sources of contamination (e.g. optimization of welding technologies to reduce particle emission).
However, even under such elaborate conditions, microcontamination may still occur at minute levels.
The current objective in purity of ultra clean electronic specialty gases to be delivered at point of use is in the range of 1 to 100 ppb (parts per billion) for any volatile impurity, particulate density lower than 1 particulate per liter (under normal conditions) and metal concentrations of less than 100 ppt (parts per trillion) for any metallic element present.
A major mechanism of microcontamination in the above considered purity range arises from the difficulty of completely removing adsorbed molecules from the surface of the materials exposed to the electronic specialty gases, and in particular of moisture, which is widespread in the environment and presents a particularly strong adsorption energy to surfaces.
It has been determined that many of the electronic specialty gases tend to react with such adsorbed molecules, particularly H.sub.2 O adsorbed on metallic surfaces, for instance, though a catalytically activated chemical reaction, which promotes the formation of volatile by-products in the considered concentration range and/or also solid particulates (see A. Bruneau et al., Abstract 30p-ZL-13, p. 54, Spring Conf. Jap. Appl. Phys, Soc., March 1994 which is hereby incorporated by reference).
The common procedure carried out to clean such a gas distribution network is to flow ultra-high purity inert gas (e.g. nitrogen or argon of purity better than 1 ppb) in order to purge all impurities present in the volume or at the interior of the gas distribution network. This procedure may nevertheless be unsatisfactory for strongly adsorbed molecules, e.g. H.sub.2 O molecules adsorbed on solid surfaces.
This purge procedure can be improved and shortened in duration by using successive pressure-vacuum cycles of the inert gas and additionally by heating the surface in order to induce thermal desorption of the strongly adsorbed molecular species. However, vacuum-pressure purge cycles alone prove inefficient in locations representing deep deadspaces because of the inefficiency of pumping through minute orifices. Thermal baking at 120.degree. C. during the purge of metallic surfaces significantly reduces the time to reach the background level of the purge gas. For example, one can obtain 1 ppb purity in a few hours when flowing ultra high purity (&lt;1 ppb impurity) nitrogen or argon at 0.1-10 standard liters per minute through a gas distribution network free of microleaks of a length between about 10 and about 200 meters. However, it is also known that the thermal desorption of H.sub.2 O on an electropolished stainless steel surface occurs in several steps, the last one being at temperature of the order of 400.degree.-450.degree. C. Such a temperature is difficult to apply in practice. Hence, when lower baking temperatures are applied, e.g. frequently 120.degree. C. for practical reasons, the metal surface is not entirely free of adsorbed moisture. Moreover, thermal baking cannot be applied in practice under some circumstances for safety, regulation or material stability reasons.
Others have attempted to improve the purge procedure of the gas distribution network, using for instance a larger number of vacuum-pressure cycles, or increasing the flow rates of the ultra high purity gases. Although this may decrease the initial concentration of the generated impurities and particulates, the time required to reach the background level is always quite long, i.e. about 40 minutes at 40 sccm flow rate and about 15 minutes at 30 sccm flow rate in the apparatus depicted in FIG. 2.
One of the problems encountered when distributing ultra high purity gases in pipes, valves, mass flow controller or the same, which are usually used in the distribution of chemical gases or other gases commonly used in the electronic industry to manufacture integrated circuits, is that those distribution systems comprise or may comprise rough surfaces, which may include deposited or adherent particulates. Even more importantly, those distribution systems usually comprise so-called deadspaces (e.g., in valves, etc.) wherein the vapor phase gas drying has not been very satisfactory.
Unexpectedly, according to the present invention and as explained hereinafter, the liquid phase processing has been proven, under certain conditions, to be much more satisfactory, particularly for rough surfaces and deadspaces encountered in the gas distribution systems such as those used in the semiconductor industry. One of the possible explanations given by the inventors (who, however, do not want to be bound by any theory) is that the wet drying agents used in accordance with the present invention may have, due to their liquid state (and the pressure needed to circulate them) not only a drying (cleaning) effect coming from their chemical formula but also a mechanical cleaning effect due to their liquid state which adds up to their chemical effect and thus provide improvement.
It is, therefore, an object of the present invention to provide another technology to more effectively clean the internal surfaces of a gas distribution network, particularly for deep deadspace locations or those which cannot be heated for thermal desorption because of practical, hazardous or regulatory conditions.
It is another object of the present invention to provide a process for the removal of adsorbed molecules, particularly H.sub.2 O, before introducing electronic specialty gases into a distribution network, in order to suppress the above source of microcontamination of volatile or particulate impurities.
Another object of the present invention is to provide a process for the operation of a gas distribution network wherein the intrusion of any moisture or air is minimized and any intruded molecules are removed.
It is yet another objective of the present invention to provide a process for minimizing contamination in the distribution of ultra high purity gases.
It is further an objective of the present invention to provide a process for minimizing particulates in the distribution of ultra high purity gases.
It is still another objective of the present invention to provide a process for the removal of H.sub.2 O adsorbed to the ultra high purity gas communication surfaces of a gas distribution network.
It is further an objective of the present invention to provide an apparatus for carrying out a process for minimizing contamination and particulates in the distribution of ultra high purity gases.